I've done it 2 ways; both are manual as our procurement system doesn't have an export function.
I started oout data entering the pertinent details in the Altiris purchase side and used the internal association function to connect things that way. It worked nicely because the data about a purchase in our procurement system is either not accurate or it's abbreviated because the fields don't allow alot of characters in them.
When we upgraded from 6.0 to 6.5 we could't import the 6.0 data into 6.5 so 2 custom fields were added to a tab in the software purchase record to record the PO# as well as an additional drop down choice list of our vendors so we could show the name of the reseller if we didn't buy direct. I also save a copy of the PO on my proof of purchase share drive and, on the purchase record's documents tab, I create an external link to it.
I use these same fields and links for documents that pre-date our procurement system where some of the proof of purchase is only a copy of an invoice or a legal document. Instead of a PO# I specify the type of document and a # or date; "invoice # 10228" or "legal document effective date Feb 16, 1997".